NEW YORK -- Dr. Chien-Shiung Wu, a physicist who
performed a historic experiment overturning what had
been considered a fundamental law of nature, died on
Sunday at St. Luke's-Roosevelt Hospital Center in
Manhattan. She was 84 and lived in Manhattan.

The cause was a stroke, according to her husband, Dr.
Luke C.L. Yuan, a retired experimental physicist.

Dr. Wu, the Michael I. Pupin Professor Emeritus of
Physics at Columbia University, where she carried out
research and taught for 37 years, was known throughout
her career as a meticulously accurate experimental
physicist who was in demand to put new theories to the
test.

In her most famous experiment, announced in 1957, she
and her colleagues overthrew a law of symmetry in
physics called the principle of conservation of parity
that had been considered immutable for 30 years. It held
that in nuclear reactions, nature in effect does not
differentiate between left and right. At one time,
physicists were so certain of the validity of the law
that they tried to make all of their observations fit
it.

But Dr. Tsung-Dao Lee, also of Columbia, and Dr. Chen
Ning Yang of the Institute for Advanced Study in
Princeton, N.J., suggested that conservation of parity
did not hold for interactions between subatomic
particles involving the so-called weak force. (One of
the four basic forces of nature, along with gravity,
electromagnetism, and the strong force, the weak force
plays a role in radioactive decay.)

Lee consulted with Dr. Wu, an expert in radioactive beta
decay, in which an atom emits electrons. Lee and Yang
suggested an experiment, but Dr. Wu had to find a way to
carry it out, an extremely difficult challenge with the
technology available at the time.

The experiment used cobalt 60 cooled to 0.01 degree
above absolute zero (or minus 459.67 degree
Fahrenheit). Dr. Wu joined forces with a research team
at the National Bureau of Standards in Washington, which
had one of the few laboratories in the country that
could chill materials to extremely low temperatures.

The cobalt 60, a radioactive isotope, was placed in a
strong electromagnetic field, which made all the cobalt
nuclei line up so they spun along the same axis.

Dr. Wu measured what happened when the cobalt nuclei
broke down, giving off electrons. She used a device that
counted the number of particles that shot out in the
direction of the spin and those that did not.

If the law of conservation of parity was correct, the
nuclei would give off equal numbers of particles in each
direction. But Dr. Wu found that far more particles flew
off in the direction opposite the spin of the nuclei,
proving that nature differentiates between left and
right.

The results were confirmed by other experiments.

Dr. Isador Rabi, a Nobel laureate in physics at
Columbia, said at the time: "In a certain sense, a
rather complete theoretical structure has been shattered
at the base, and we are not sure how the pieces will be
put together."

Lee and Yang were awarded the Nobel Prize in Physics in
1957 for their theoretical work in this area. Dr. Wu did
not share the prize, but the playwright Clare Boothe
Luce said at the time, "When Dr. Wu knocked out that
principle of parity, she established the principle of
parity between men and women."

Lee said on Monday: "C.S. Wu was one of the giants of
physics."

Dr. Wu was born on May 29, 1912, near Shanghai. She
received a bachelor's degree from National Central
University in Nanjing in 1936 and a doctorate from the
University of California at Berkeley in 1940.

She taught at Smith College and Princeton University
before being asked to join the Manhattan Project, the
Army's secret project to develop the atomic bomb in
World War II. She helped develop a process to enrich
uranium ore that produced large quantities of uranium
235, a fuel for the bomb.

As part of the project, an atomic pile was constructed
at Hanford, Wash., in 1942, but its chain reaction
stopped a few hours after it was started. The physicist
Enrico Fermi suspected that a substance produced by
nuclear fission was halting the reaction by capturing
most of the neutrons.

He was reportedly told: "Ask Miss Wu." It turned out
that a rare gas that she had studied in graduate school
was responsible, and the problem was cleared up.

After the war, Dr. Wu joined Columbia as a research
associate. She was named a full professor in 1958 and
was appointed the first Pupin Professor of Physics in
1973.

In her research, she obtained the first successful
measurements of low-energy electrons emitted by beta
decay, providing experimental evidence for Fermi'
theory of weak interactions in the nucleus. She was an
author of "Beta Decay," published in 1965, a standard
reference for nuclear physicists.

In 1975, Dr. Wu became the first woman to be elected
president of the American Physical Society, the chief
organization of physicists in the United States. She wa
also the first woman to receive the Cyrus B. Comstock
Award of the National Academy of Sciences and the first
woman to receive an honorary doctorate in science from
Princeton. The university's president, Robert Goheen,
said in presenting it in 1958 that Dr. Wu had "richly
earned the right to be called the world's foremost
female experimental physicist."

She also received the National Medal of Science, the
nation's highest award for achievement in science, and
the Wolf Prize in physics. Dr. Wu was elected to the
National Academy of Sciences, and was elected to the
Royal Society of Edinburgh as an honorary fellow.

Besides her husband, she is survived by a son, Vincent
Yuan, of Albuquerque, N.M., who is a research scientist
at Los Alamos National Laboratory.